Where a larger resolution indicates a better separation of peaks.[1][2] This definition is used in a number of mass spectrometry texts.[3][4][5][6][7][8][9][10][11] This use is also implied by the term "high-resolution mass spectrometry."[12]

A high value for resolution corresponding to good separation of peaks is similar to the convention used with chromatography separations,[13] although it is important to note that the definitions are not the same.[14] High resolution indicating better peak separation is also used in ion mobility spectrometry[15]

There are several ways to define the minimum peak separation ΔM in mass spectrometry, therefore it is important to report the method used to determine mass resolution when reporting its value.
The two most widely used are the peak width definition and the valley definition.[1]

In the peak width definition, the value of ΔM is the width of the peak measured at a specified fraction of the peak height, for example 0.5%, 5%, 10% or 50%. The latter is called the full width at half maximum (FWHM).

The valley definition defines ΔM as the closest spacing of two peaks of equal intensity with the valley (lowest value of signal) between them less than a specified fraction of the peak height. Typical values are 10% or 50%. The value obtained from a 5% peak width is roughly equivalent to a 10% valley.[1]